The Antarctic is known for its snow and ice covered mountains, its grand ice shelves, and its frozen embayments, and today (21 May), we had the pleasure of working in the latter and having distance views of the former in remarkably good weather. We are now finishing work in the embayment east of Charcot Island where work was conducted in a large expanse of sea ice scattered with ice bergs and at the sea ice edge. Our current position at 2026 local time is -70 18.591°S; -75 41.079°W. Winds are out of the east (091) at less than 10 kts and the air temperature is -3.8°C
On the 19th of May, the weather continued to be cloudy and although the winds were in the less than 20 knot range for most of the morning, by late afternoon they were up around the 30 kt range and the working conditions became marginal. Still no work was canceled. On 20 May, the weather was quite good for a change. Some blue sky was even showing for a portion of the daylight hours. Winds were light and the temperature hovered around freezing. In the early afternoon, the broad-scale survey was ended at Station 84. The last activity was a MOCNESS tow (#18). From the bridge around noon during the tow, Charcot Island was visible to the east with its black rock surfaces showing where it was not covered with a mantle of snow and ice. Also barely visible was an ice shelf that extended out into the sea away from the island to the southeast. The end of the survey also brought an end to the systematic way in which the scientific effort was conducted. The list of tasks to be done in the next ten days is large, but when and where they will be accomplished depends to a large extent on finding the right conditions. The first priority was to find an area with penguins and whales that could be approached using the Zodiac inflatable boats. To this end, we decided to steam into an embayment east of Charcot Island, which offered some protection from the prevailing northeast wind and was a likely place for some pack ice. On the way to the embayment east of Charcot Island, the CTD group took the opportunity to define the coastal current structure and hydrography a bit more by taking two CTDs along the way, including one at a location occupied by Stan Jacobs in 1994, and a series of XBTs. At the Jacobs location, patches of ice chunks coalesced around the ship and when the CTD got to the surface, one soccer ball sized piece of ice came aboard on top of the Rosette. By mid-night as we steamed into the bay from the Jacobs' location, many large grounded (?) icebergs visible on radar and the sea ice was thickening.
During the past two days, work was completed at Stations 79 to 84 of the survey and two additional stations on the way to the embayment east of Charcot Island including eight CTDs, five XBTs, five sonabuoy deployments, three ring net and two MOCNESS tows, two sets of bird and one set of whale observations, and BIOMAPER-II towyos between each of the stations.
Eileen Hofman reports that on 19 May the continuing good weather conditions allowed the CTD and XBT work to continue as scheduled for the day. CTD casts were completed at survey stations 79, 80, 81, and 82. In between these stations we dropped XBT probes. At the end of the day all of survey transect twelve had been completed. The quality of the data obtained from the CTD casts and XBT drops remains high. This is in large measure due to the excellent technical support provided by Raytheon MTs Matt Burke and Dave Green and ETs Jeff Otten and Jan Szelag. Their efforts are most appreciated.
The distribution of the temperature maximum below 200 m shows that the coastal current flowing outward from the southern portion of Marguerite Bay turns and flows in a southwesterly direction along the west Antarctic Peninsula continental shelf. The current remains as a coherent alongshore feature for about 100 km. The across-shelf extent of the current is about 60 km, almost reaching the outer portion of survey transect twelve. At about 40 to 50 km alongshore, the coastal current appears to dissipate and to show meandering and possibly eddy formation along its southern-most boundary. The vertical profiles of temperature and salinity at the individual survey stations show evidence of considerable mixing which is consistent with the coastal current breaking down. The across-shelf extent and dissipation of the coastal current are interesting because the prevailing wind patterns are such that the current should remain as a narrow band that flows to the southwest along the inner shelf after it exits Marguerite Bay. Understanding the dynamics of this current will have to await further data analysis and numerical circulation models.
Susan Howard has produced ADCP-derived current distributions along all of the survey transects. The current patterns at the surface and deeper are consistent with the flow patterns suggested by the maximum temperature distribution. The current vectors indicate that flows associated with the coastal current are on the order of 5-8 cm/sec. Flows associated with the meandering portion of the coastal current are higher, about 15 cm/sec.
Nutrient samples continue to be collected at all CTD stations by Howard
Rutherford and Rebecca Conroy. The patterns seen in the nitrate and silicate
distributions, which are good markers for Circumpolar Deep Water, are consistent
with the flow suggested by the hydrography and ADCP-derived currents. We
are now plotting horizontal distributions of these nutrients at specific
depth levels for comparison to the hydrographic distributions.
Bob Beardsley's Met Notes May 19: Surface Cooling - Part 2
In Met Note May 3, we described the surface forcing during the first six days of the large-scale physical/biological survey when the NBP made five cross-shelf transects before entering Marguerite Bay. The preliminary results were: a) the mean wind and wind stress (0.11 N/m2 were directed towards 132T (SE), b) the mean surface heat flux Qnet = -108 W/m2, with about 95% of this heat loss being due to the net upward longwave flux, and c) the contributions of the shortwave, sensible, and latent heat flux components were quite weak due to persistent cloud cover, the low air temperatures, and the air-sea temperature difference being small.
In this note, we will examine the surface forcing during the last five days (May 14-18) when the NBP has made four cross-shelf transects off Alexander Island after leaving Marguerite Bay. The first transect was made on May 14 during a gale, with winds towards the south and peak winds for several hours above 40 kts. This storm bought relatively warm air over the shelf, such that during the strongest winds, the sensible and latent heat fluxes were positive into the ocean, and the net surface flux Qnet was slightly positive for three hours (above 20 W/m2 for almost 2 hrs). This was the only time of surface warming during this five-day period. Winds during the remaining transects were mostly south- and southwestward and more moderate. The air temperature dropped during this second period, but remained above the ocean surface temperature, so that there were weak sensible and latent heat fluxes into the ocean. These fluxes were too weak, however, to balance the large longwave heat loss, so that Qnet was negative for the remaining transects. The shortwave contribution to the surface heat flux was very small, due primarily to the more southern location of these transects made later in the month, and the persistent overcast.
The mean atmospheric conditions and surface forcing during the gale
(May 14) and the next four days (May 15-18) are summarized in the following
Table 1A. Mean atmospheric conditions during May 14 (gale)
|Vector wind speed||16.0 m/s|
|Vector wind dir||-95.1 deg (counterclockwise from E)|
|Scalar wind speed||16.5 m/s|
|Sea surface temp||-1.15°C|
Table 1B. Surface forcing during May 14 (gale)
Table 2A: Mean atmospheric conditions during May 15-18
|Vector wind speed||10.1 m/s|
|Vector wind dir||-130.5 deg|
|Scalar wind speed||11.4 m/s|
|Sea surface temp||-1.24°C|
Table 2B: Surface forcing during May 15-18
These tables suggest the following conclusions about the surface forcing during the cross-shelf transects made May 14-18:
a) Winds were generally strong and almost always southward, generating southward wind stresses that varied from near zero to about 0.8 N/m2 (8 dynes/cm2).
b) The combination of very strong winds and warm air from the north (during the gale) can produce large enough sensible and latent heat fluxes into the ocean that the net surface heat flux can be positive into the ocean for a short time.
c) Excluding the gale, the net surface heat was negative, cooling the ocean. The small sensible and latent fluxes into the ocean (due to the air being warmer than the ocean surface by ~ 0.5°C and mean winds near 20 kts) are more than offset by the roughly constant longwave heat loss (Qlw ~ -80 W/m2), resulting in a mean Qnet ~ -70 W/m2.
These results support the basic picture of persistent surface cooling
over the shelf driven by net longwave radiation loss. The shortwave flux
becomes negligible with time and moving further southward, and the sensible
and latent fluxes are generally small except during high wind/warm air
events (e.g., the May 14 gale). The persistent overcast reduces the net
longwave cooling, but it remains the dominant process in the surface heat
Susan Howard on the Acoustic Doppler Current Profiler (ADCP):
The GLOBEC cruises are using Acoustic Doppler Current Profiler (ADCP) measurements, in conjunction with other direct current measurements, to determine the current structure in the Marguerite Bay region and along the western Antarctic Peninsula continental shelf. The ADCP, mounted on the hull of the ship, estimates the water velocity along a cruise track. It therefore provides a spatial overview of the velocity field in the study region.
The ADCP uses the Doppler effect to measure the relative velocity between the instrument and scatterers in the ocean. The ADCP measurements are based upon acoustical signals or pings (1 ping per second) that are sent into the water column from a four element transducer array beneath the ship. The transducer emits acoustical energy out at 150 kHz in four beams. The ping encounters scatterers, such as plankton and other small particles (assumed to be moving with the water), which create an echo. The echoes are reflected back up towards the ship and the ADCP records these return echoes. Echoes from shallow depths return sooner than the echoes from deeper depths. The ADCP sorts and averages the return echoes into separate depth bins. Scientists determine the number of bins and size of bins based upon what they want to see. For the GLOBEC cruises, the ADCP on the Palmer is configured to use 50 bins (8 meters in depth), which gives us good spatial resolution and depth coverage. The ADCP uses the Doppler effect to convert the return echoes of 3 beams into velocity components (the 4th is redundant and is used to check the others). The binned velocity data from the 1 second ping is then averaged over time (1-10 minutes) to reduce noise. The resulting ensemble averaged data provides the velocity of the water relative to the ship at specific depth bins. The ADCP provides good velocity estimates down to about 350 m (depending on conditions).
Since the ship and water are both moving, the ensemble averaged data must be processed to get to absolute velocities. The processing involves editing the data (checking for bad data) and calibrating the data (adjusting the data due to errors in the instrument alignment). The final step requires the use of navigation data to remove the ship's velocity (obtained from GPS data) from the relative ADCP velocities in order to get true, or absolute, water velocities.
By examining the absolute velocities, we have been able to identify
several interesting features in the survey so far. A coastal current of
5-8 cm/s flows southward along the coast of Adelaide Island and enters
Marguerite Bay from the North. Examination of the deeper circulation patterns
suggest that deep currents enter Marguerite Bay and form 2 circulation
cells. Meanders in the Antarctic Circumpolar Current on the edge of the
shelf break are also identifiable. All this information will be combined
with the other direct current measurements, as well as the hydrographic
measurements, to give scientists a clear picture of the basic circulation
patterns of this region.
Chris Ribic and Erik Chapman report that on 19 May they surveyed for
3 hours and 17 minutes between stations 80 and 81. They recorded few birds
during this survey over the outer portion of the continental shelf. Antarctic
and Snow Petrels were the most common birds seen. Here's a summary of today's
|Species||Number 3hr 17min|
|Antarctic Petrel (Thalassoica antarctica)||9|
|Cape Petrel (Daption capense)||1|
|Southern Fulmar (Fulmarus glacialoides)||6|
|Blue Petrel (Halobaena caerulea)||0|
|Southern Giant Petrel (Macronectes giganteus)||0|
|Snow Petrel (Pagrodoma nivea)||8|
They did not survey on 20 May because we arrived at station 84, the
final station on the grid, before sunrise.
Catherine Berchok reported that on May 18th, she deployed sonobuoy #45 on the way back to Station 76 from the MOCNESS. Faint humpback sounds could be heard in the 400 Hertz range over a 2.5 hour listening period. Sonobuoy #46 was thrown in 5 miles from station 77. A faint humpback song in all frequency ranges was heard for another 2.5 hours. Bearings to the sounds were calculated from a couple of calls, and put the animals in the general direction of Station 69.
Sonobuoy # 47 was deployed at 6 nm from station 78. A very loud song was heard from at least two nm, with four to five different singers heard at one point. The hearing of repeating patterns of sounds, implies whale song. Bearings on the sounds indicate that there are whales in at least two separate locations, one in line with Stations 77 and 78 and another in line with the 78-79 line. Recordings were made from this buoy for 4.5 hours
Sonobuoy #48 was deployed at 19.5 nm from station 79 in order to get a cross bearing on the sounds from buoy #47. Unfortunately, the carrier signal on #47 went goofy and although the whale sounds were quite loud, it was impossible to get a fix on their position. Time delays and differences in signal strengths will make it possible to get a general idea of the source bearings. Recording was for one hour.
Sonobuoy # 49 was deployed about a half-hour later when we were off far enough to get a cross-bearing to the #48 buoy - about 15 nm to station 79. Of course it turned out to be the first sonabuoy of the trip that failed to transmit any signal. A couple of computer crashes ensued and by the time everything was back in order, we had moved into the faint range of the humpback sounds - so that was it for the day.
So it seems as though there are still a few male humpbacks along the
coast here. In addition to the humpbacks, a minke grunt or two may also
have been heard.
BIOMAPER-II/MOCNESS report (P. Wiebe, C. Ashjian, S. Gallager and
BIOMAPER-II continued to run without difficulty from any of its sensor systems on May 19 and 20. It was in the water, except for the period when it was brought on deck to allow for a MOCNESS tow at Stations 81 and towyos were made between each of the stations (79 to 84). One improvement to the data logging system was made early on 19 May. At long last, the problem in getting RS232 measurements of the Dynacon winch readouts (wire tension, meters of wire out, and wire payout/in rate) to a logging computer were solved by Scott Gallager.
The last two transect lines provided additional evidence that the pattern of high acoustic backscattering from the coast to near the outer margin of the continental shelf and then much lower backscattering in the offshore waters is continuous throughout the entire survey region. The scattering intensities along the most southerly sections were generally lower than in the region just south of Marguerite Bay. This area seemed to be dominated by copepods, especially below 100 m, and fewer krill were observed.
Two MOCNESS tows were done during 19 and 20 May, one (#17) at Station 81 (to 500 m) and the other (#18), at Station 84 (to 460 m) near the middle of the continental shelf. The eight nets sampled intervals varying from 50 m to 150 from the bottom to 100 meters and then 25 meter intervals from 100 m to the surface. The zooplankton collections were dominated by copepods, but the biomass levels in the samples were low compared to collections further to the north. Fewer krill were caught in the nets.